As emission regulations become more stringent, engines increasingly rely on Exhaust Gas Recirculation (EGR) to reduce NOx. EGR systems recirculate a portion of engine exhaust gas back into the combustion cylinders.
NOx is extremely harmful to the human body and is mainly produced in a high-temperature and oxygen-rich circumstance. The higher the combustion temperature is and the longer the time duration is, the more the NOx product is. In the operation of the engine, if a part of the exhaust gas is directed into the cylinder again timely and moderately, a part of heat produced by combustion is absorbed and brought out the cylinder through the exhaust gas, and the intake gas is diluted by the exhaust gas to some degree, since a main component CO2 of the exhaust gas has a large heat capacity. Thus, a highest temperature and an oxygen content of the engine combustion are reduced, and thereby the NOx compounds are reduced.
In a supercharged and intercooled diesel engine, a main way to perform the exhaust gas recirculation is to direct the exhaust gas, located at front of a turbine, into an intercooler, which is called high-pressure exhaust gas recirculation. In order to ensure a stable EGR rate in different operation conditions, the best way is to adopt a variable geometry turbocharger, in which a difference between a pressure PT before the turbine and a pressure PK after the intercooling is adjusted by changing the geometry of the turbocharger for driving an EGR valve.
Most of current variable geometry turbochargers are added a rotatable nozzle blade in the turbine. The communication of a channel of the turbine is changed by changing an opening degree of the nozzle blade, which is easy to control. However, since the exhaust gas discharged from the engine has a high temperature of about 700 degrees Celsius and the temperature may be getting higher, the high temperature has proposed strict requirements on the nozzle blade, a transmission mechanism, a nozzle ring support disc and an external control system, and, impurities in the high-temperature exhaust gas discharged from the engine pose a reliability risk on a complex moving component.
Moreover, the variable geometry turbocharger is only applied in the field of high-end engine supercharging due to an expensive price. Therefore, marketing for such a product is limited in terms of cost and reliability. In addition to the above main factors, there are other technical problems.
The first problem is that: when a flow rate is reduced, the opening degree of the nozzle blade is required to be reduced, thus a circumferential speed of the turbine intake gas increases, the turbine becomes an impulsive impeller, which does not facilitate making full use of exhaust gas energy, and thus an efficiency for the turbine is low, and an exhaust back pressure of the engine high.
The second problem is that: when the opening degree of the blade is small, a flow path for the turbine intake gas is increased, and flowing loss is increased, thus the nozzle blade is over far away from the turbine, and the gas flow mixing loss is increased.
The third problem is that: a gap must present at both ends of the blade, so as to facilitate the rotation of the blade, however, this gap may cause leakage loss and reduce efficiency of the turbine.
Another widely applied variable geometry application is a two-layer flow channel variable geometry turbocharger developed by Kangyue Technology CO., LTD. By designing the structure of the traditional turbine housing, a combination of internal and external two-layer intake channels is adopted to form different flow passage cross sections. Therefore, all functions of the variable geometry turbine can be achieved by effective usage of the exhaust gas energy of the engine in different sections without providing a complex rotary vane pneumatic regulation mechanism, and thus the cost is greatly reduced, the reliability is greatly improved, and this will become a research direction in the future.
A structural principle of such a product is as shown in FIG. 1, when the engine operates at a low speed and a low load, the supercharging pressure is low, and a regulating valve 5 is closed, that is, only an inner intake flow channel 3 is communicated in a turbine housing 1, and the gas flow flows along a flowing direction 4 of an inner intake flow channel so as to allow the turbine to work.
As shown in FIG. 2, when the supercharging pressure reaches a certain degree, a control actuator 7 pushes a valve pin piece assembly 6. The valve pin piece assembly 6 opens the regulating valve 5. The inner intake flow channel 3 and an outer intake flow channel 2 work simultaneously, the gas flow flows along the flow direction 4 of the inner intake flow channel flowing direction 4 and a flow direction 8 of the outer intake flow channel, so as to allow the turbine to work. However, there are a lot of problems in this structure in an actual fitting testing and application process.
The problems are described as follows:
firstly, when the engine operates at a high speed, a large amount of exhaust gas is discharged, and in order to ensure high-speed performance and avoid overspeed of the engine due to the turbine pressure at high-speed operation, the cross sectional area of the two-layer flow channel is relatively large; in terms of the cross sectional area allocation for the two-layer flow channel, low-speed performance and the EGR rate of the engine are affected if the inner intake flow channel has a relatively large cross sectional area; and an cross sectional area difference between the inner intake flow channel and the outer intake flow channel is large if the inner intake flow channel has a small cross sectional area, thus, transient expansion loss is too large in the case that the regulating valve is opened, and it is difficult to control the engine at a middle speed under medium-load;
secondly, the flow rate and the pressure cannot be controlled accurately through a simple valve structure shown in FIGS. 1 and 2 in the middle speed of the engine under the medium-load phase, further, control deviation of the flow rate and the pressure will not only affect matching performance, but also affect the EGR rate, which results in excessive engine emissions;
thirdly, multiple vortex tongues are formed by formation of the inner intake flow channel and the outer flow intake channel, and gas flows are easy to be affected by each other, which results in the reducing of the efficiency of the turbine; and
fourthly, the structure of the turbine provided with the inner and outer flow channels is complex, and the turbine is also a casting part, therefore, producing consistency is difficult to control, and if the casting deviation of the two flow channels is large, the effect on the matching performance of the turbine is also large.